1. Field of the Invention
The invention relates to a process for the preparation of ethanol.
2. Background
The preparation of ethanol by catalytic hydration of ethylene is known, for example, from Hydrocarbon Processing & Petroleum Refiner, November 1963, Volume 42, No. 11, page 162. In the method described in said article a liquid condensate is obtained by cooling down the effluent gas mixture from the reactor. After having been scrubbed with water the noncondensed gases are recycled to the reactor and the condensate and scrubbing liquid are combined. The resultant mixture consists of an aqueous solution of ethanol containing diethyl ether and acetaldehyde as principal impurities. The ethanol is steam-stripped from the aqueous solution, together with the impurities. The impure alcohol is passed over a nickel catalyst with hydrogen, in which treatment, inter alia, the acetaldehyde is converted into ethanol. Subsequently, in the purification section of the plant the low-boiling by-products, mainly diethyl ether, are removed overhead from a distillation column and a mixture of ethanol and water of substantially aceotropic composition is obtained by fractional distillation of the bottom product in a second distillation column.
In an article by T. C. Carle and D. M. Stewart in Chemistry and Industry, 12th May, 1962, pages 830-839, various by-products are enumerated that can be formed in the hydration of ethylene. From the above-mentioned acetaldehyde, crotonaldehyde can be formed under certain conditions by aldol condensation followed by dehydration. Further, higher olefins with up to 8 carbon atoms, in particular butylene, are formed by polymerization of ethylene. Higher alcohols can be formed from said olefins by reaction with water. One of said alcohols is butanol-2, which can be dehydrogenated into methyl ethyl ketone in the reactor.
It is not simple to prepare pure ethanol from crude alcohol in an economically justified manner. In the methods described in the above-mentioned articles the crude ethanol is catalytically hydrogenated. In this treatment the aldehydes and ketones are converted into the corresponding alcohols. The higher alcohols can be separated from ethanol more readily than the aldehydes. These methods still have the disadvantage that the hydrogenation of large quantities of crude ethanol is less attractive for technical and economic reasons and, therefore, other methods have been developed.
In German patent application DAS No. 1 903 552 a purified mixture of ethanol and water of about azeotropic composition is obtained from the crude aqueous ethanol by distillation in three columns. In said process the aqueous crude ethanol containing both lower-boiling and higher-boiling impurities is distilled extractively in a first distillation column, water being added at the top of the column. In this column the greater part of the impurities, both the higher- and the lower-boiling impurities, distills overhead. As bottom product aqueous ethanol with a concentration of about 5-10% is obtained which is purified and concentrated in a second distillation column. The purified ethanol-water azeotrope is obtained as a side stream from the upper part of the second column. The lower-boiling impurities distill overhead from said column and the higher-boiling impurities leave the column as one or more side streams. The top products of the first and second column and the impurities-containing side stream or side streams of the second column are introduced into a third distillation column. From the upper part of the third column an ethanol-rich side stream is obtained that is recycled to the first colunn. Higher-boiling impurities are discharged as one or more side streams and lower-boiling impurities, such as diethyl ether and acetaldehyde, are removed from the system via the top of the third column. It appears from German patent application DAS No. 1 903 552 that the diethyl ether and acetaldehyde-containing top stream of the third column still mainly consists of ethanol. This loss of ethanol, however, is considered as inevitable. Further, the method of German patent application DAS No. 1 903 552 has the disadvantage that aldehydes, such as acetaldehyde and crotonaldehyde are ultimately present throughout the purification section of the plant. This has an unfavourable effect on the quality of the ethanol.
German patent application DAS No. 2 106 073 (U.S. equivalent being U.S. Pat No. 3,960,672 issued June 1, 1976) recommends that aqueous caustic lye be added to the second distillation column of the above-mentioned process below the point where ethanol is collected but above the point where the higher-boiling impurities leave the column, and that the bottom product of the second column, after neutralization, be discharged or introduced into the upper part of the first column. This results in the recovery of ethanol of better quality with mainly a lower acetaldehyde content and a better permanganate time. However, this method has the disadvantage that in the system a gradual increase takes place of inorganic and organic impurities (polymers) causing corrosion and pollution. Further, accurate neutralization of the caustic lye requires the use of rather complicated apparatus. Further, this process has also the disadvantage that the top stream to be removed from the third distillation column contains considerable quantities of ethanol, diethyl ether and acetaldehyde. It appears from Examples 2 and 3 of German patent application DAS No. 2 106 073 that the ethanol can be recovered by means of extractive distillation with water in a fourth distillation column and recycled to the first distillation column. However, the process is cumbersome and not very economic and, moreover, the resultant aqueous ethanol will again contain acetaldehyde owing to the high solubility of acetaldehyde in water. With regard to the ether recovered in the extractive distillation, said patent application only mentions that it can serve as raw material for producing ether. Consequently, the ether is apparently still highly contaminated. In this connection it is noted that the distillative separation of diethyl ether and acetaldehyde is greatly impeded by azeotrope formation.
German patent application DAS 2 545 508 (equivalent to U.S. Pat. No. 3,990,950 issued Nov. 9, 1976) also states that the process according to German patent application DAS No. 2 106 073 has the disadvantage of excessive use of lye and neutralizing acid. Even if the aqueous bottom product of the second column is not recycled to the first column, the alkali present should be neutralized before the bottom stream can be discharged, in order to prevent environmental pollution. However, drawbacks are also involved in the discharge of water containing large quantities of salts. Therefore, in the published German patent application DAS No. 2 545 508 the quantity of caustic lye that is added to the second column is kept within specified low limits. Consequently, ethanol with a lower acetaldehyde content is obtained, but it is obvious that the above-mentioned drawbacks were not eliminated